The PG-13 and R rating split is lowering the quality of films. The movie industry is forced to cater to 13 year old teens instead of a more mature 15 and up target demographic, the largest one that sees most big blockbuster movies.
Due to these constraints for teenagers the rules get really silly, like the one F word limit on PG-13 films, not allowing any type of nudity and limiting violence to cartoon depictions.
There are exceptions to this rule, some movies tend to adjust to the PG-13 rating and still achieve darkness and grittiness, but the compromise to art for a PG-15 demographic would be considerably less substantial.
The UK has a good example of this system, they have 12, 15, and 18. This would be an easy and extremely beneficial change to the MPAA rating system